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[sql-over-http] Reset session state between pooled connection re-use (#12681)

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Session variables can be set during one sql-over-http query and observed
on another when that pooled connection is re-used. To address this we
can use `RESET ALL;` before re-using the connection. LKB-2495

To be on the safe side, we can opt for a full `DISCARD ALL;`, but that
might have performance regressions since it also clears any query plans.
See pgbouncer docs
https://www.pgbouncer.org/config.html#server_reset_query.

`DISCARD ALL` is currently defined as:
```
CLOSE ALL;
SET SESSION AUTHORIZATION DEFAULT;
RESET ALL;
DEALLOCATE ALL;
UNLISTEN *;
SELECT pg_advisory_unlock_all();
DISCARD PLANS;
DISCARD TEMP;
DISCARD SEQUENCES;
```

I've opted to keep everything here except the `DISCARD PLANS`. I've
modified the code so that this query is executed in the background when
a connection is returned to the pool, rather than when taken from the
pool.

This should marginally improve performance for Neon RLS by removing 1
(localhost) round trip. I don't believe that keeping query plans could
be a security concern. It's a potential side channel, but I can't
imagine what you could extract from it.

---

Thanks to
https://github.com/neondatabase/neon/pull/12659#discussion_r2219016205
for probing the idea in my head.
This commit is contained in:
Conrad Ludgate
2025-07-23 18:43:43 +01:00
committed by GitHub
parent 5c57e8a11b
commit a695713727
10 changed files with 161 additions and 68 deletions
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28
libs/proxy/tokio-postgres2/src/client.rs
View File

@@ -292,8 +292,32 @@ impl Client {
simple_query::batch_execute(self.inner_mut(), query).await
}
pub async fn discard_all(&mut self) -> Result<ReadyForQueryStatus, Error> {
self.batch_execute("discard all").await
/// Similar to `discard_all`, but it does not clear any query plans
///
/// This runs in the background, so it can be executed without `await`ing.
pub fn reset_session_background(&mut self) -> Result<(), Error> {
// "CLOSE ALL": closes any cursors
// "SET SESSION AUTHORIZATION DEFAULT": resets the current_user back to the session_user
// "RESET ALL": resets any GUCs back to their session defaults.
// "DEALLOCATE ALL": deallocates any prepared statements
// "UNLISTEN *": stops listening on all channels
// "SELECT pg_advisory_unlock_all();": unlocks all advisory locks
// "DISCARD TEMP;": drops all temporary tables
// "DISCARD SEQUENCES;": deallocates all cached sequence state
let _responses = self.inner_mut().send_simple_query(
"ROLLBACK;
CLOSE ALL;
SET SESSION AUTHORIZATION DEFAULT;
RESET ALL;
DEALLOCATE ALL;
UNLISTEN *;
SELECT pg_advisory_unlock_all();
DISCARD TEMP;
DISCARD SEQUENCES;",
)?;
Ok(())
}
/// Begins a new database transaction.